PR #21231 made user topic creation rely on system topic `__change_event` if the user is enabling `topicLevelPoliciesEnabled`.

It will introduce a race condition with namespace bundle unloading. All creating topics want to create `__change_event` reader but constantly fail by namespace-bundle inactive and retry mechanism. Unfortunately, the namespace-bundle unloading operation is waiting for all the topics to be completed and then release inactive status. Therefore, they will be stuck in a deadlock until one gets a timeout.

- Get the topic policy before loading.

(cherry picked from commit 428c18c)